Conventionally, as power factor improving converters, systems using the boost chopper circuits are well known. FIG. 15 shows a circuit diagram of this system. A diode 21, a diode 22, a diode 23, and a diode 24 are bridge-connected, an input thereof is connected to an AC power source 1, and an output thereof is connected to a series circuit including a choke 2 and a MOSFET 37. A series circuit including a diode 25 and a capacitor 54 is connected between a source and a drain of the MOSFET 37, and a load 3 is connected to both ends of the capacitor 54.
Since the power factor improving converter receives an input current obtained by a filter (not shown) removing a high frequency component from a current of the choke 2, a control is performed so that a low frequency component of the current of the choke 2 becomes similar in waveform to the voltage of the AC power supply 1, thereby realizing the power factor improving function.
The current of the choke 2 can be controlled by turning on and off the MOSFET 37. When the MOSFET 37 is turned on, a voltage of the MOSFET 37 becomes zero. When the MOSFET 37 is turned off, the diode 25 becomes conducted. Therefore, the voltage of the MOSFET 37 becomes equal to a voltage of the capacitor 54, that is, an output voltage.
Accordingly, an equivalent circuit of FIG. 15 focusing on a change in voltage of the choke 2 will be a circuit shown in FIG. 16. Here, a variable voltage source 4 has a value of ±mVo where Vo represents the output voltage. m has a value of 0 or 1, a sign will be positive when the voltage of the AC power supply 1 is positive, and the sign will be negative when the voltage of the AC power supply 1 is negative. Here, a state where an upper side of the AC power supply 1 is higher in potential than a lower side thereof is defined as a positive voltage, while the reverse state is defined as a negative voltage.
Since this circuit is a boost chopper, it is premised that the output voltage is higher than the input voltage. Therefore, when m=0, that is, the MOSFET 37 is turned on, the current of the choke 2 increases. When m=1, that is, the MOSFET 37 is turned off, the current of the choke 2 decreases. It is possible to control the current of the choke 2 by controlling a ratio of the on-off, thus enabling a control such that the input current of the power factor improving converter becomes similar in waveform to the voltage of the AC power supply 1.
On the other hand, a circuit of FIG. 17 is known as a circuit that realizes the same operation as the above. Here, when the same components as those of the circuit shown in FIG. 15 are represented by the same symbols, a MOSFET 38, a MOSFET 39, a MOSFET 40, and a MOSFET 41 are bridge-connected, an input thereof is connected to the AC power supply 1 via the choke 2, an output thereof is connected to a capacitor 54, and the load 3 is connected to both ends of the capacitor 54.
It is apparent in the circuit shown in FIG. 17 that when all the MOSFETs are turned on, it becomes equal to the state where m=0 in the circuit shown in FIG. 16. Additionally, when all the MOSFETs are turned off, body diodes of the respective MOSFETs constitute the bridge diode, and its rectifying action makes the circuit equal to the state where m=1 in the circuit shown in FIG. 16. Accordingly, similarly to the circuit shown in FIG. 15, it is possible to make the circuit shown in FIG. 17 function as a power factor improving converter.
Additionally, two of the MOSFET 38, the MOSFET 39, the MOSFET 40, and the MOSFET 41 are replaceable with diodes, such as known as circuits shown in FIG. 18 and FIG. 19.
It is apparent in the circuit shown in FIG. 18 that when the MOSFET 40 and the MOSFET 41 are turned on, it becomes equal to the state where m=0 in the circuit shown in FIG. 16. Additionally, when the MOSFET 40 and the MOSFET 41 are turned off, the diode 26, the diode 27, the body diode of the MOSFET 40, and the body diode of the MOSFET 41 constitute the bridge diode, and its rectifying action makes the circuit equal to the state where m=1 in the circuit shown in FIG. 16. Accordingly, similarly to the circuit shown in FIG. 15, it is possible to make the circuit shown in FIG. 18 function as a power factor improving converter.
In the circuit shown in FIG. 19, if the MOSFET 39 is turned on while the AC power supply 1 is at the positive voltage, a current flows in a route from the choke 2 via the diode 26 to the MOSFET 39, it becomes equal to the state where m=0 in the circuit shown in FIG. 16. Additionally, if the MOSFET 41 is turned on while the AC power supply 1 is at the negative voltage, a current flows in a route from the MOSFET 41 via the diode 28 to the choke 2, it becomes equal to the state where m=0 in the circuit shown in FIG. 16. Further, if the MOSFET 39 and the MOSFET 41 are turned off, the diode 26, the diode 28, the body diode of the MOSFET 39, and the body diode of the MOSFET 41 constitute a bridge diode, and its rectifying action makes the circuit equal to the state where m=1 in the circuit shown in FIG. 16. Accordingly, similarly to the circuit shown in FIG. 15, it is possible to make the circuit shown in FIG. 19 function as a power factor improving converter.
By the way, a method of increasing a switching frequency is common in order to miniaturize such a power factor improving converter. By increasing the switching frequency, an inductance of the choke required to achieve the same ripple current decreases, thereby enabling the miniaturization of the choke.
However, disadvantages caused by increasing the switching frequency include an increase in switching loss, an increase in choke copper loss due to the increase in AC resistance of the choke coil, and an increase in choke iron loss due to the high frequency characteristics of the core. Since the increase in loss causes an increase in size of cooling components, there has been a problem that the miniaturization effect of the power factor improving converter achieved by increasing the switching frequency reaches a plateau.
Additionally, there is another problem that the conventional power factor improving converter has large common mode noise. The common mode noise is generated by a common mode current flowing into the ground. However, this common mode current is generated by a change in potential which occurs at the time a switch element such as a MOSFET is switched. In the case of MOSFETs, although the back of the element becomes a drain, the common mode current flows into the ground through a stray capacitance present between the drain and the ground. When iC represents the common mode current, CSTRAY represents the stray capacitance, and dV/dt represents a time variation of the drain voltage with respect to the ground,
                              i          C                =                              C            STRAY                    ×                                    d              ⁢                                                          ⁢              V                                      d              ⁢                                                          ⁢              t                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Accordingly, in order to reduce the common mode current iC, three methods can be considered, such as reducing the stray capacitance CSTRAY, reducing dV, and increasing dt. If an insulator present between the drain and the ground is thickened in order to reduce the stray capacitance CSTRAY, however, there is a problem that the thermal resistance increases, thus causing an increase in temperature of the MOSFETs. dV is not changeable because it is determined from the circuit configuration that dV=±Vo. Additionally, when dt is increased, the switching loss increases, thus causing a problem that the temperature of the MOSFETs increases.
Thus, since it is not easy to reduce the common mode current, such a forcible measure as increasing the impedance of the noise filter is taken in some cases. In order to increase the impedance, however, there is a problem such that an expensive material has to be used, or a noise filter becomes larger in size.